1. Field of the Invention
The present invention relates to a coating apparatus and method for applying coating material onto a web supported by a backing member. More particularly, the present invention relates to a coating apparatus and method for applying pressurized coating material onto a web of paper.
2. Information Disclosure Statement
In the web coating art, coating material is applied to a rotating roll and offset onto an application roll such that a web contacting the application roll takes up a portion of the coating material. However, such coating apparatus tends to saturate the web with coating material so that excessive quantities of coating material are applied.
More recently, the so-called short dwell coater has proved very successful in reducing the amount of coating material absorbed by the web when compared with the aforementioned prior art pond coating methods.
Essentially, the short dwell coater includes a housing defining an application zone supplied with a source of pressurized coating material. A web of paper moves past an open face of the application zone while supported by a backing member, such as a backing roll. A doctor blade is disposed immediately downstream relative to the application zone for removing excess coating material applied to the web through the open face of the application zone. Consequently, the dwell time of the web relative to the coating material in the application zone is minimal so there is very little time for the coating material to be absorbed by the web.
Nevertheless, with a short dwell coater, the pressure at which the coating material is supplied must be relatively low in order to prevent excessive flow over the baffle, that is the leading wall, and loss of coat weight control due to pressure on the blade, sometimes exhibited as streaking of the resultant coated web.
U.S. Pat. No. 5,010,840 to Busker et al proposed a short dwell coater having an overflow weir between the application zone and the doctor blade such that excess coating material could be recirculated.
However, in certain applications, it has been desirable to apply to the coating material a greater pressure than that possible with the aforementioned short dwell coater.
Accordingly, the present invention relates to an apparatus and method of applying a coating material to the surface of the sheet using a shoe with coating material supplied to the leading edge of the shoe and to a cavity near the center of the shoe. The shoe can be followed by a blade if required in accordance with the coating weight to be applied to the web.
The advantages of the aforementioned arrangement include the development of pressures in the application zone that are much higher than can be developed in a short dwell coater head while eliminating the disadvantages of the prior art coating ponds and the like where vortexing within the coating material is a problem.
The present invention provides a means for applying coating material to sheet material and eliminates air entrainment and vortexing of the coating material within the application zone, thereby providing coating uniformity at even higher speeds.
More specifically, the application of coating material to a web using the short dwell coating head has many advantages. Among such advantages are good runnability and clean operation. However, at operating speeds above 3,000 feet per minute, machine directional streaks and mottling appear on the applied coating. Such streaks are thought to be the result of vortexing that develops in the pond or application zone of the short dwell coater head.
The aforementioned offset application roll technique, otherwise known as "flooded nip coating", uses a roll to apply the coating to the sheet and has less of a problem with streaks. However, the flooded nip coater includes other problems, such as non-uniformity of the resultant coating material and the lack of cleanliness.
The present invention provides means for applying coating to the sheet using an application shoe, thereby eliminating the development of vortices in the pond. Additionally, the present invention gives a measure of control over the thickness of the coating film being applied to the sheet.
By mounting the shoe such that it is able to move freely towards and away from the web, the runnability of the arrangement is preserved.
Additionally, by keeping the shoe relatively small and in a confined space, an increase in cleanliness during operation of the apparatus is achieved.
Therefore, it is a primary objective of the present invention to provide a coating apparatus which overcomes the aforementioned inadequacies of the prior art arrangements and which makes a considerable contribution to the art of coating material onto a paper web.
Another object of the present invention is the provision of a coating apparatus which includes a shoe for eliminating the development of vortices in an application zone.
Another object of the present invention is the provision of a coating apparatus which, in use thereof, exhibits enhanced cleanliness.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description taken in conjunction with the annexed drawings.